Selective circuits for voice frequency carrier telegraph systems



SELECTIVE CIRCUITS FOR VOICE FREQUENCY CARRIER TELEGRAPH SYSTEMS Filed March 2, 1931 FIG. 2

FIG. I

Llfifdk. KELLEY BY p ATTORNE l a 3 S S s 2 l 3 R R R 0 )L T 3 L N n w M I m F T l 2 3 C C C I k A 59 R H8 F6 2 F 0 70 W A w I N W F -m O 5 O O O 5 O m 5 2 I. .I 3 2 2 Patented Jan. 23, 1934 UNITED STTE GFFICE' Leo A. Kelley, Jackson Heights, N. Y., assignor to International York Communications Laboratories, Inc., New York, N. Y.

, a corporation of New Application March 2, 1931. Serial No. 519,415

5 Claims.

My invention relates to improvements in selective circuits; especially electrical circuits designed to operate upon predetermined frequencies for either the transmission of energy or the reception thereof.

An object of my invention is to provide, for use mainly in connection with electric signalline. a type of selective circuit that is simple but efiicient, easy to set up, and that lends itself quite readily to the requirements of various signalling channels with regard to frequency, impedance and other characteristics.

A further object is to provide a selective circuit comprising sections linked one to another in order to give the degree of selectivity desired.

Another object is to provide a selective circuit which is made up largely of interchangeable members of the same construction and dimensions, so that diiferent electrical characteristics are obtained chiefly by mere adjustment, and the need for many parts of special design is thus obviated.

An additional object is to provide selective circuits so arranged that they permit the easy insertion of means for controlling the energy level therein when said circuits are used, without disturbing the impedance conditions necessary for the most eflicient operation.

Other objects of my invention are set forth in the description taken with the accompanying drawing which illustrates the manner in which the invention is practiced.

Referring to the drawing;

Fig. 1 shows double tuned circuits arranged so that one reenforces the efi'ect of the other to increase the selectivity of the circuits;

Fig. 1a is a view showing the transmission characteristics of this type of circuit;

Fig. 2 is a view showing selective circuits of the same character as Fig. 1 but with a still larger number cooperating to give increased selectivity;

Fig. 2a is a view showing the transmission characteristics of this type of circuit of Fig. 2; and

Fig. 3 indicates in outline a signalling system comprising transmission and receiving channels containing selective circuits according to this invention.

On the drawing the same numerals identify the same parts throughout.

Fig. 1 shows a circuit comprising an inductance coil 1 in series with a condenser 2. Cooperating with this circuit is a similar circuit comprising an inductance coil 3 in series with a condenser 4. The coils 1 and 3 are magnetically coupled; and by choosing coils 1 and 3 of the same inductance and the condensers 2 and 4 of the same capacity, the same resonant frequency is secured for these two circuits. The value of this frequency is, of course, determined by the value of the capacity of condensers 2 and 4; the amount of inductance of the coils 1 and 3. If now a generator or other source of electrical energy having an output impedance equal to the circuit impedance is connected to theterminals 5 and 6 of the coil and condenser 1 and 2; and areceiver of input impedance equal to the same circuit impedance is connected to the terminals 7 and 8, the response of the receiver to various frequencies from the generator will be in ac-- cordance with the selectivity characteristic for which the circuit is designed. I

The arrangement of Fig. 1 may be regarded as a unit or section of which as many can be joined together as is required. To insert a unit of this type into a working circuit and secure optimum results, this unit should have characteristic impedance equal to that ofthe circuit into which it is included. It can easily be shown that with a given circuit impedance, the value of the inductance of the coil 1 for a given transmitting band width can be computed according to methods which have been Worked out. By selecting the transmission band in cycles per second and by taking the circuit impedance in ohms, the value of theinductance-in'henries can easily be calculated. With, the transmitting band width selected and the resonant frequency known, the co-efiicient of coupling to insert between the coils 1 and 3 to secure this band width can also be obtained. Likewise from the resonant frequency and the value of the-inductance, the capacity of condenser 2, and that of the condenser 4 being the same, is arrived at.

To give a concrete example, suppose the operating impedance of the circuit to which the terminals 5 and 6 are connected is 600 ohmsand the band width is to be 100 cycles. The inductance of the coils 1 and 3 can easily be ,found to be henries. Hence, if it is desired to work on a resonant frequency of 1000 cycles per second, the capacity of the condenser 2 and also of the condenser 4 can be found to be the damping factors of the coils and condensers and the limitations of a double tuned circuit. The characteristics of a tuned circuit of this type are shown in Fig. la for frequencies of 800, 1000 and 1200 cycles per second, and transmission curves A, B and C in this figure are seen to be alike in form, signifying that for different frequencies, the transmission characteristics of the circuit will be the same.

The selectivity of this circuit can be increased as follows:

If a second pair of double tuned circuits is joined to the first pair as shown in Fig. 2, this extra pair of double tuned circuits comprising the coils 9 and 11 in series with the condensers 10 and 12 respectively, the selectivity of the arrangement will be increased; the inductances of the coils 9 and 11 being made equal to the inductances of the coils 1 and 3, the coils 9 and 11 being given the same coupling and the capacities of the condensers 10 and 12 being the. same as that of the condensers 2 and 4. The connection of two more units such as shown in Fig. 1, in the manner as indicated in Fig. 2, gives a triple tuned circuit, one containing the coil 1 and condenser 2, another containing the coil 11 and condenser 12, and the third being the intermediate link which couples the first two together. This intermediate link is made up by uniting the terminals of condensers 4 and 10 and the terminals of the coils 3 and 9. With the same constants as before; that is, with the inductances of the various coils the same as in the first case, the couplings the same, the capacities the same and the same band width and resonant frequency, the transmission characteristics for selective circuits of this sort are illustrated in Fig. 2a. The curves are here represented at A, B and C for transmission on a band width of 100 cycles at 800, 1000 and 1200 cycles per second respectively.

It will be noted that any interfering signals lying outside the transmitted frequency range will now be attenuated to an increased amount. But transmission and reception can be obtained equally Well on any one of the resonant frequencies indicated.

It being shown by a comparison of Fig. 1a with Fig. 2a that discrimination against the unwanted signals has been increased simply by increasing the number of tuned circuits in cascade, it is evident that it is only necessary to include the proper number of these tuned circuits to secure the right degree of selectivity for a practical signalling system such as is indicated, for example, in Fig. 3. Here, a main line L is connected to a number of transmitting channels T1, T2 and T3. These transmitting channels contain selective circuits C1, C2 and C3, such as has been described, together with signalling devices K1, K2 and K3; and at the other end of the line are receiving channels R1, R2 and R3 containing sclective circuits S1, S2 and S3 respectively. A larger number of transmitting and receiving channels can be established; each transmitting channel Working upon a frequency of its own which is different from the frequencies of the other channels and the receiving channels each being tuned to receive messages from one of the transmitting channels only. This diagram represents a carrier current telegraph system with carrier current frequencies of say 800, 1000 and 1200 cycles for the three transmitting and receiving channels and a transmitting band width of 100 cycles. When more transmitting and receiving channels are added, frequencies'of above and below those named will be employed.

To adapt the selective circuits so that they will operate upon different resonant frequencies, the

value of the inductances of the coupling coils need not be changed, the capacities are altered and the couplings between the different coils are adjusted; the co-efiicient of coupling being smaller for the higher frequencies and larger for the lower frequencies. Hence, for different resonant or working frequencies, the same band widths and the same values of inductance can be chosen. This method of designing selecting circuits gives very important practical advantages. It is customary when a selecting circuit of a certain band width and a certain degree of selectivity is desired, to construct and utilize a so-called wave filter containing series and shunt arms consisting of condensers and inductances. If the correct impedance relationships are to be obtained, a different filter for each transmitting and receiving channel is required. In other words, the filter for each of the channels working on different resonant frequencies must be specially designed. Ihus, different type inductance coils must be provided and the cost of the circuit is greatly increased. For the selecting circuits shown in Figs. 1 and 2, only one type of inductance coil is needed for the entire system.

The various inductance coils 1 and 3, 9 and 11 are preferably, but not necessarily in the form of multi-layer coils with air cores and of rectangular cross-sections. To obtain the proper coupling, which, as above stated, is selected according to the frequency and band width desired, the coils are disposed so as to be movable with respect to each other. They may be adjusted by moving one or the other or both in any direction, keeping the coil axes parallel, or by rotating them about an axis at right angles to the coil axes, and in other ways. These coils are all mounted in the same manner on the panels in the various channels so that the correct co-efiicient of coupling can be readily obtained when the system is set up.

The selecting circuits herein described are also adapted to permit the insertion of devices for controlling the energy levels in the various channels and thus to allow for attenuation in transmission, which is unequal. Such a device can be connected as shown at 13 and 14 on Fig. 2 without disturbing the impedance conditions of the circuit or diminishing any of the advantages or altering the transmission characteristics already described.

With selective circuits of the kind set forth, a multi-channel carrier telegraph circuit can be inexpensively manufactured and installed. Because all of thecoils in these circuits can be alike in size, number of turns, resistance, size of wire, etc., any one of the coils can be used in connection with any one of the channels, because the coupling between any two of the coupled coils need only be adjusted to suit the frequency without altering the inductance and resistance, and the only things that need changing, as above stated, are the capacities of the condensers, such capacities being made inversely proportional 'to the square of the resonant frequency. This relation is obtained from the known equation when fis the resonant frequency, L is the inductance, and C the capacity of the resonant circuit. Hence, in installing this system, like coils can be connected in all of the channels and adjusted afterward to give the correct coupling.

The selecting circuits are described in connection with a carrier current telegraph system. They are, of course, not limited in this way, but may be employed with various other signalling systems.

While I have described two units in accordance with Figs. 1 and 2, it is to be understood that when additional selectivity is desired, and also to maintain the other characteristics such as band width, circuit impedance, etc., it is only necessary to connect additional units of Fig. 1.

What is claimed is:

l. The method of producing selectivity in electrical circuits containing inductance coils and condensers responsive on a predetermined band width at a particular resonant frequency, which consists in assigning to said coils the same value of inductance, coupling said coils in pairs and adjusting the coupling to suit said frequency and band width, and regulating the capacity of said condensers according to said frequency.

2. The method of producing selectivity in electrical circuits containing condensers and inductance coils responsive on a predetermined band width at a particular resonant frequency, which consists in assigning to said coils equal values of inductances which are independent of the frequency of the channel and are a function of the band width and circuit impedance, regulating the capacity of said condensers according to the resonant frequency desired and adjusting said coils so as to make the co-efiicient of coupling inversely proportional to said resonant frequency.

3. The method of producing selectivity in electrical circuits containing coils and condensers being responsive on a predetermined band width at a given resonant frequency, which consists in coupling portions of said circuits together, fixing the inductance of said coils at a common value independent of said frequency and proportional to said band width and circuit impedance, arranging the coupling of said coils so that the co-efficient of coupling is inversely proportional to said frequency and regulating the said condensers so that the capacities thereof are inversely proportional to the square of said frequency.

4. The method of producing selectivity in electrical circuits containing coils and condensers and being responsive on a predetermined band width at a given resonant frequency which consists in taking units or sections comprising at least one coil and condenser, linking said units or sections together according to the required number, regulating the capacity of the condensers in accordance with the resonant frequency, fixing the inductance of said coils at the same value throughout the circuits, making said inductance dependent upon the band width and independent of said frequency, and adjusting the coupling between said units or sections so that the co-efiicient of coupling is inversely proportional to the resonant frequency of said circuit.

5. Selective circuits comprising inductance coils and condensers for operating over a limited band width at a predetermined resonant frequency, said circuits comprising sections, each containing at least one inductance coil and a condenser, the sections being linked together in the required number to give desired selectivity, said condensers being adjustable according to the resonant frequency desired and said units or sections being coupled to a degree inversely proportional to the resonant frequency. and said coils all having a fixed value of inductance.

LEO A. KELLEY. 

